The invention relates to an apparatus and method for separating solids from a solids laden liquid and particularly, but not exclusively an apparatus and method for separating solids from a solids laden drilling mud.
In the drilling of a borehole in the construction of an oil or gas well, a drill bit is arranged on the end of a drill string, which is rotated to bore the borehole through a formation. A drilling fluid known as “drilling mud” is pumped through the drill string to the drill bit to lubricate the drill bit. The drilling mud is also used to carry the cuttings produced by the drill bit and other solids to the surface through an annulus formed between the drill string and the borehole. The density of the drilling mud is closely controlled to inhibit the borehole from collapse and to ensure that drilling is carried out optimally. The density of the drilling mud affects the rate of penetration of the drill bit. By adjusting the density of the drilling mud, the rate of penetration changes at the possible detriment of collapsing the borehole. The drilling mud may also carry lost circulation materials for sealing porous sections of the borehole. The acidity of the drilling mud may also be adjusted according to the type of formation strata being drilled through. The drilling mud contains inter alia expensive synthetic oil-based lubricants and it is normal therefore to recover and re-use the used drilling mud, but this requires inter alia the solids to be removed from the drilling mud. This is achieved by processing the drilling mud. The first part of the process is to separate the solids from the solids laden drilling mud. This is at least partly achieved with a vibratory separator, such as those shale shakers disclosed in U.S. Pat. No. 5,265,730, WO 96/33792 and WO 98/16328. Further processing equipment such as centrifuges and hydrocyclones may be used to further clean the mud of solids. The solids are covered in contaminates and residues. It is not uncommon to have 30 to 100 m3 of drilling fluid in circulation in a borehole.
The resultant solids, known herein as “drill cuttings” are processed to remove substantially all of the residues and contaminates from the solids. The solids can then be disposed of in a landfill site or by dumping at sea in the environment from which the solids came. Alternatively, the solids may be used as a material in the construction industry or have other industrial uses.
Shale shakers generally comprise an open bottomed basket having one open discharge end and a solid walled feed end. A number of rectangular screens are arranged over the open bottom of the basket. The basket is arranged on springs above a receptor for receiving recovered drilling mud. A skip or ditch is provided beneath the open discharge end of the basket. A motor is fixed to the basket, which has a drive rotor provided with an offset clump weight. In use, the motor rotates the rotor and the offset clump weight, which causes the basket and the screens fixed thereto to shake. Solids laden mud is introduced at the feed end of the basket on to the screens. The shaking motion induces the solids to move along the screens towards the open discharge end. Drilling mud passes through the screens. The recovered drilling mud is received in the receptor for further processing and the solids pass over the discharge end of the basket into the ditch or skip.
The screens are generally of one of two types: hook-strip; and pre-tensioned.
The hook-strip type of screen comprises several rectangular layers of mesh in a sandwich, usually comprising one or two layers of fine grade mesh and a supporting mesh having larger mesh holes and heavier gauge wire. The layers of mesh are joined at each side edge by a strip which is in the form of an elongate hook. In use, the elongate hook is hooked on to a tensioning device arranged along each side of a shale shaker. The shale shaker further comprises a crowned set of supporting members, which run along the length of the basket of the shaker, over which the layers of mesh are tensioned. An example of this type of screen is disclosed in GB-A-1,526,663. The supporting mesh may be provided with or replaced by a panel having apertures therein.
The pre-tensioned type of screen comprises several rectangular layers of mesh, usually comprising one or two layers of fine grade mesh and a supporting mesh having larger mesh holes and heavier gauge wire. The layers of mesh are pre-tensioned on a rigid support comprising a rectangular angle iron frame and adhered thereto. The screen is then inserted into C-channel rails arranged in a basket of a shale shaker. An example of this type of screen is disclosed in GB-A-1,578,948 and an example of a shale shaker suitable for receiving the pre-tensioned type screens is disclosed in GB-A-2,176,424.
Drilling rigs often have space restrictions for mud processing equipment, particularly, but not exclusively on off-shore drilling rigs. Shale shakers have a foot print defined by a skid. Instead of having separate scalping shakers and primary screening shakers, shale shakers may incorporate a scalping deck and a primary screening deck to reduce the number of skids required on a drilling rig. An example of such a shale shaker is the VSM 300® supplied by NATIONAL OILWELL VARCO®. Shale shakers may include multiple primary screening decks inter alia to screen more solids laden drilling fluid per skid. Furthermore, primary, secondary and tertiary decks each with different screening material thereon may be incorporated on a single skid to inter alia provide a filtered drilling fluid with finer solids therein or to filter out graded solids for re-circulating in the well.
Solids laden drilling fluid may have chemicals therein which give off noxious fumes, fumes detrimental to the environment and fumes which may potentially be explosive. Such fumes may include hydrogen sulphide and oil vapours. Accordingly, it is often desirable to remove or contain such fumes. It is also advantageous to provide a separator which operates at low noise levels to comply with health and safety legislation. It is also advantageous to have a simple reliable separator to inhibit down-time for maintenance and repair.
It is advantageous to use fine-meshed filters to filter very small particles, for example of a size in the range of 50-200μ or more, without the filtering device clogging up with the small particles. However, it is the fine-meshed filters in particular that are prone to such undesired clogging. It is preferable in certain circumstances to retain particles, for example of a particle size in the range of 50-60μ or larger, by means of a filter.
It is known to house shale shakers in rooms on or near derricks of a drilling rig. These rooms are known as shaker rooms. A ventilation system may be used in such shaker rooms to draw off fumes given off by the solids laden drilling fluid when being processed by the shale shakers.
US 2008/0078699 discloses a shale shaker for separating solids from solids laden drilling mud, the shale shaker comprising a basket with a fume hood arranged thereon.
WO 2006/098638 discloses an apparatus for separating solids from solids laden drilling fluid, the apparatus comprising an inlet screen and upper and lower horizontal endless belt screen cloths energized with acoustic pulses, the endless belt screen cloths having a screen cleaning system using jets of air and the endless belt screen cloth having a discharge end and a negative pressure ventilation system thereabove.
The inventors have observed that the quantity of fumes given off by the shale shaker is inter alia related to the screening area of the screens in the shale 15 shakers. Thus with the popularization of shale shakers with multiple-decks, more fumes are given off per skid. The inventors also observed that there is a need for a more efficient apparatus and method for drawing off fumes produced by the shale shakers.